This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The nitrogenase enzyme system catalyzes the essential process of biological nitrogen fixation. It is composed of the MoFe protein, where substrate reduction occurs, and the Fe protein, which is the specific electron donor to the MoFe protein. FeMoco, a [MoFe7S6X] cluster in the MoFe protein, is believed to be the site of nitrogen catalysis. The central low-Z atom, X, at the core of FeMoco was recently discovered by crystallography (O. Einsle, et al. Science 297, 1696 (2002)). The presence of an interstitial atom in FeMoco raises new questions about the structure, biosynthesis, and reactivity of this cluster that we hope to address using single crystal XAS. Polarized single crystal XAS has the advantage of providing both local structural and electronic detail about a metal site in a protein that is related to a known protein tertiary structure and, further, can be used to selectively enhance specific molecular vectors through crystallographic alignment with the beam polarization vector. We have preliminary results that indicate the feasibility of applying this technique to interrogate the central atom of FeMoco. The aim of this study is to further develop and utilize single crystal XAS for the study of important questions in the nitrogenase system including (1) detection and quantification of an interstitial atom in FeMoco;(2) determination of FeMoco structure, especially with regard to the presence of an interstitial atom, in the absence of N2 catalysis through the study of a catalytically inactive MoFe protein;(3) correlation of the Fe-Fe distances in the all-ferrous Fe protein with the global protein structure to gain an understanding of how this distinct cluster may function in catalysis;and (4) elucidation of the mechanism of substrate and inhibitor interaction with FeMoco through the study of radiolytically reduced, substrate-incubated MoFe proteins.